It is well known that pseudocolor enhances human perception of gray scales and enables an observer to quantify a single parameter image. However, it is also known that most medical radiologists do not like pseudocolor images.
Electronic Picture Archiving and Communication Systems (PACS) provide a convenient mechanism for storing multicolor images and ultrasound flow visualization systems now demonstrate the necessity for color displays when more than one independent parameter must be simultaneously displayed. It has been shown that color can allow the eye-brain combination to form useful correlations on multiparameter image data, if it can be used in a pleasing manner.
It is known that separate use of red, green and blue to image three independent variables leads to confusing images. A more natural presentation is used in commercial color television and in map making where a primary image is shown as a high (spatial) resolution intensity image and secondary parameters are visualized by color tinting (which typically has less spatial resolution than the primary image). Thus, the hue (color) and the saturation (purity of color) can be used to present two independent low-resolution variables which are superimposed on a high resolution intensity image.
FIG. 1 schematically illustrates a two-dimensional chromaticity space of hue and saturation in polar coordinates. In the figure, S=0 is white (zero color saturation) and the circle S=1 represents pure monochromatic colors (fully saturated). In this model, locations near S=0 are pastel colors.
In the prior art, three parameters of position in an image (such as a(x,y), f(x,y) and g(x,y)) were respectively assigned to Intensity, Hue and Saturation as, for example: EQU Intensity: I(x,y)=K.sub.i a(x,y) (1) EQU Hue: H(x,y)=K.sub.h f(x,y) (2) EQU Saturation: S(x,y)=K.sub.s g(x,y) (3)
where K.sub.i, K.sub.h, and K.sub.s were constants.
The simplest use of these relations is to set S=0 everywhere (which implies that the hue is irrelevant) black and white intensity images are thus produced from the function a(x,y).
In the usual prior art pseudocolor display, the single parameter, I(x,y) is set to a constant, S is set to unity (fully saturated color), and the parameter f(x,y) is imaged with hue as the only variable. Many people find such images distasteful.
In a more acceptable prior art method, "color tinting" of a gray scale image is accomplished using all three equations. If only one additional parameter is of interest, it is common to set S equal to a constant and to use H(x,y) as the parametric variable. Thus color tinting of a black and white image conveys low resolution information as an overlay through which the observer can see the usual image of intensity information a(x,y) in the same manner as color tinted black and white photographs. If S is chosen as unity, this scheme is still unsatisfying to many observers. More pleasing images have been formed if S is set to a small value which leaves all colors as unsaturated pastels.
In medical ultrasound, conventional pulsed-echo instruments display a monochrome image of the envelope of the RF echo signal which is returned from the body. Isolated point scatterers and large, reflecting interfaces thus show up as points or lines. The image represents a slice which is perpendicular to a plane swept out by the ultrasound beam. The resolution cell is formed by the RF pulse length (in the axial direction) and the diffraction limited lateral focus of the transducer aperture after any propagation distortions (attributable for example to refractive index variations in the body).
Living human tissue possesses a significant frequency dependent attenuation characteristic which is normally modeled, (to a first approximation) in healthy liver tissue, as linear with a slope of approximately 0.5 db/cm/MHz. As an example, at 3 MHz, the attenuation for a "normal" human liver tissue is 1.5 db/cm in each direction or 3 db/cm for round trip pulse echoes. A 20 cm path depth thus requires a time-gain-compensation (TGC) amplifier with a 60 db gain increase over the time of the return signals after the transmit pulse. When a broadband RF pulse is transmitted, lower frequency components of the signal are attenuated less than the high frequency components. Thus, the spectral centroid (and hence the average instantaneous frequency) of the echoes shifts downward as a function of the depth of the reflector.
It has been hypothesized that diseased livers have physical structures which may scatter and attenuate ultrasound energy in different manner than healthy tissue. Numerical tissue characterization methods have attempted to measure the attenuation down-shift separately from gross tissue structures and to avoid non-homogeneous regions of tissue (such as central portions of the liver).